There's been a lot of hype around intermittent fasting for fat loss over the past few years, and it's increasingly rising in popularity across the internet and social media platforms. But is it just a new weight loss fad, or is fasting the magic fat-loss solution we've all been looking for? Thankfully with the rise in interest, intermittent fasting has extended past mainstream media and into the research world.
What is Intermittent Fasting?
Intermittent fasting is a dieting approach where an individual's eating window is limited to a specific period - either days or hours - and they fast for the remainder of the time. While this approach is straightforward, there are dozens of ways to implement it. Below are a few of the more common fasting methods.
Time-restricted eating is when people fast for a specific number of hours in a day and consume calories during designated feasting hours. The most popular fasting methods include the 12:12, 14:10, and 16:8. Within these methods, people fast for 12, 14, or 16 hours respectively, and consume calories for 12, 10, or 8 hours.
OMAD (One Meal A Day) is self-explanatory. An individual fasts for 23 hours and consumes calories within a 1-hour eating window.
The 5:2 diet is a less extreme form of intermittent fasting, where the individual eats regularly for five days and restricts their calories by 500-600 calories for two days.
Alternate-day fasting is when people eat regular calories every other day and heavily restrict their calories on alternating fast days. The original version recommends zero calories on fasting days, while the revised version allows 500 calories on fasting days.
Another fasting approach suggests eating zero calories for one day per week and regular calories or time-restricted eating on the other days.
Does Intermittent Fasting Work for Fat Loss?
Intermittent fasting is an effective means of fat loss because it forces people to reduce their caloric intake. There are claims that intermittent fasting targets fat cells more efficiently during fasting periods, but no evidence supports this claim. Overall, the research shows that if you fast without controlling caloric intake, participants lose little weight, if any at all. When comparing higher-frequency meals to intermittent fasting, participants showed no significant difference in weight loss. So, there's no "magic" in fasting - fat loss will only occur if an energy deficit exists.
Research for the Popular 16:8 Approach:
Most human studies are relatively short, ranging from 4 days to 12 weeks, with 8-80 participants in total. These studies point to a calorie deficit as a means of weight loss, with participants in fasting groups losing the same amount of weight as those with higher meal frequency in a calorie-controlled environment. A majority of studies spanning over one year were animal-based. Longer-term studies with humans and more prominent group sizes are still needed to decipher if intermittent fasting is more or less advantageous than calorie restriction.
Thankfully, Liu et al. (2022) conducted a 12-month randomized clinical trial to assess time-restricted eating (16:8 approach) with calorie restriction for weight loss and metabolic risk factors in 139 obese patients. Researchers instructed all participants to maintain a 25% calorie deficit: women's caloric consumption at 1200-1500 calories and men's at 1500-1800 daily. The participants also received health coach counseling. They were encouraged to weigh/log their food and photograph their daily meals for the first six months. The second half of the experiment urged participants to follow the same meal plan. Still, it only required three days of weighing/logging/photographing their meals. There was no indication of a specific exercise regimen or recommended protein intake. Study results appear below.
97.1% completed the first six months (135 participants), and 84.9% for the entire 12-month experiment (118 participants).
Participants in the time-restricted eating group adhered to their diet 84% of the time, and the calorie-restrictive group was at 83.8%, on average.
There was no significant difference between the two groups at 6 or 12 months of weight loss. The net difference ranged from -4 to .4 kg.
The participants in the calorie restriction group lost slightly more throughout the experiment.
Those who lost 5%, 10%, and 15% of their body weight were similar between both groups.
Waist circumference and BMI were similar between both groups.
All participants lost total body fat and lean mass. The two groups had no significant difference, with a net difference ranging from −3.1 to 0.2 kg.
There was no significant difference between the two groups when testing for blood pressure, lipid profile, glucose/insulin, and other cardiometabolic risk factors.
Researchers did not report any serious adverse effects.
The study concluded that intermittent fasting is no more advantageous than calorie restriction.
Is Intermittent Fasting Detrimental to Muscle Retention?
Obese individuals hold onto their muscle mass more effectively in an energy deficit than leaner individuals. Also, this study did not require the participants to include resistance training. So, we had to look at more research to determine if intermittent fasting adversely affects muscle retention in hypocaloric conditions.
A study by Tinsley et al. (2019) investigated the effects of time-restricting eating with the added component of resistance training using an 8-week mixed trial model. In the intention-to-treat (ITT) model, forty active women were split into three feeding groups: controlled diet (no designated eating window), time-restricted eating, and time-restricted eating with supplements. Along with a slight calorie deficit, the women received a resistance training program to follow 3x per week while consuming 0.7g of protein per body weight. The other model used the per-protocol (PP) framework, including twenty-four active females with the same caloric deficit, 3x per week resistance training, and protein set at 0.7g/lb. Researchers found no significant differences in body composition between the groups using the ITT model. But found significantly different results in fat mass using the PP model, where the women in the controlled diet group (calorie restricted) showed higher levels of fat loss.
The following year, Stratten and colleagues conducted a 4-week study to determine if time-restricted eating has an added benefit of muscle retention, as highlighted in the media. They placed participants in a 25% calorie deficit and randomly separated them into calorie-restricted and time-restricted eating groups. Both groups had resistance training programs to execute and a protein requirement of 0.8g/lb. The findings showed that time-restricted eating does not enhance fat loss, and both groups maintained their lean muscle mass.
Alan Aragon, a nutritional researcher, was asked about muscle retention during intermittent fasting. He said:
"The body's surprising capability of muscle-preservation in hypocaloric conditions is apparent despite suboptimal feeding patterns/low meal frequency, as long as totals are met & resistance training is sound."
However, Aragon added that intermittent fasting is "not ideal for the goal of maximizing muscle growth."
In other words, if you're getting sufficient protein to spare muscle loss and training hard enough, it doesn't matter if you eat three or more meals daily or consume all your calories during a specific eating window. But restricting calories by spreading your meals throughout the day is a better way to maximize your gym efforts.
Intermittent Fasting and Disordered Eating:
According to an article published by Medical New Today, The Canadian Study of Adolescent Health Behaviors surveyed 2,762 young adults and adolescents to assess disordered eating behaviors and body image issues. Researchers pulled their data and found a greater connection with disordered eating among the participants who engaged in intermittent fasting within 12 months, specifically binge eating, laxative use, compulsive exercising, self-induced vomiting, and body dysmorphia. Several other studies also link fasting during many hours of the day and developing episodes of overeating and binge eating disorder. Intermittent fasting may also normalize abnormal eating behaviors, allowing starvation and lack of portion control to be socially acceptable and part of a healthy lifestyle.
Drawbacks to Intermittent Fasting:
More long-term studies with larger sample sizes are needed to determine muscle retention and fat loss in a hypocaloric state compared to calorie restriction.
The human-based studies lacked follow-up with the participants.
Intermittent fasting may create disordered eating behaviors, such as binge eating and compensatory behaviors.
Studies over one year were primarily on animals.
Fasting may cause individuals to eat more than their regular calories, defeating the purpose of weight loss and associated health benefits.
Intermittent fasting is another way to restrict caloric intake—a recommendation to count calories will help avoid overeating.
Intermittent fasting may cause intense hunger pangs, headaches, sleep disturbances, malnutrition, and dehydration.
Workout performance and cognitive abilities may suffer with a fasting diet.
Overall, intermittent fasting does not have the "magic" that the media portrays. Its health benefits are associated with fat loss by creating a calorie deficit. Keeping protein high should still be maintained to promote muscle retention. However, the standard method of calorie restriction may be more advantageous for those looking to build skeletal muscle and improve body composition.
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